Method of bleaching semichemical pulps



Patented Oct. 31, 1950 UNITED STATES PATENT OFFICE Robert L. McEwen, Williamsville, N. Y., assignor to Buffalo Electro-Chemical Company, Inc.,

Tonawanda, N. Y.

No Drawing. Application August 13, 1947, Serial No. 768,507

1 Claim.

The present invention relates to a method of bleaching wood pulp containing a considerable proportion of ligneous matter and more particularly to a method of bleaching so-called semichemical pulps.

In a co-pending application, Serial Number 765,647, filed August 1, 1947, a continuation of Serial No. 572,255, filed January 10, 1945, now Patent No. 2,465,327 of March 22, 1949, there is described a method of bleaching mechanical wood pulp, that is to say, wood pulp containing substantially all of the ligneous matter associated with the wood from which the pulp was made. I have now discovered that a similar procedure may be applied to the bleaching of semi-chemical pulps.

Semi-chemical pulps are pulps still containing a major portion of the lignin initially contained in the wood. From about to 45% of the lignin has been removed by mild digestion with limited amounts of chemicals, the wood thereafter being mechanically disintegrated. Like mechanical pulp, semi-chemical pulp is a high yield pulp based upon the weight of wood initially fed to the pulping equipment.

The semi-chemical pulp to be treated in accordance with the present invention may be prepared in any conventional fashion. That is to say, the logs are fed to a chipping machine as contrasted with a grinder, and the chips so produced are digested in any of the usual chemical operations, that is, subjected to a neutral sulfite cook or to a mild digestion in accordance with either the soda or sulfate chemical digestion operations. Whereas in the usual production of chemical pulp, it is the desire to remove 100% of the lignin, in semi-chemical operations, it is desired to remove only a minor amount of the lignins. The pulps so produced are not of sufficiently high grade to be used in the production of the highest quality papers but are blended with quality fibers in varying proportions depending upon the demand of the trade and may be used alone for some purposes. The difficulty of obtaining a permanent bleach of pulp containing lignin, that is to say, a bleach which does not revert with ageing, has militated against the employment of any large proportion of lignin containing pulps heretofore, either alone or in blends where permanently bright stock is desired and, in the production of quality materials, it was necessary to employ so-called full chemical pulps wherein the pulp yield might vary from about 40 to 55%, depending upon the type of wood initially employed and upon the cooking conditions to pH 9.0, more or less.

It is an object of the invention to provide a bleaching method for semi-chemical pulps.

It is also an object of the invention to provide a procedure for bleaching wood pulps which contain residual lignin to the extent of about 7 to 15% of the dry weight of the pulp.

It is also an object of the invention to provide a combined hypochlorite and alkaline peroxide bleach upon wood pulp containing considerable amounts of lignin.

In accordance with the present invention, a semi-chemical pulp, that is to say, one containing a large proportion of the initial lign l and, therefore, containing from about 7% to 15% of lignin by weight, based upon the weight of the oven dried pulp, is initially conditioned or bottomed by treating the pulp in an aqueous environment and in the presence of carefully controlled quantlties of active chlorine as, for instance, a water soluble hypochlorite and under relatively rigidly controlled conditions of hydrogen ion concentration.

The environment in which the pulp is treated may be slightly acid, neutral or slightly alkaline, provided only that the pH of the environment in which the active chlorine initial treatment is performed is in the range from about pH 3.5 In general, it will be preferred to perform the active chlorine treatment within the pH range between about pH 5 to pH 7.5. Generally, alkali metal hypochlorites or chlorine and caustic alkali are the preferred reagents for generating the active chlorine, although alkaline earth metal hypochlorites, or other water soluble hypochlorite, may serve this purpose. In some cases, chlorine gas may be bubbled through the slurry, provided suitable agitation and hydrogen ion control are obtained, and provided the chlorine be not employed in so great quantity as to attack and remove the lignin.

The amount of active chlorine to be employed is regulated within very narrow limits, such amount being only a relatively small proportion by weight, dry basis, of the pulp to be treated and being also insufilcient to produce soluble chlorlignin. The active chlorine concentration in the pulp suspension, calculated on the basis of dry pulp, may vary from 0.1% to 1.75%. The preferred concentration is 0.25% to 1.0% active chlorine; the pulp is then bleached with alkaline peroxide. When semi-chemical pulp is treated under these conditions, the yield of bleached pulp is substantially of the amount of semichemical pulp fed to the process.

The initial conditioning or bottoming action may be performed upon the pulp at any desired consistency although generally the consistency of the pulp will be in the neighborhood oi 1-6% .more or less, that is to say, usual mill consistencies, although the process is operable at higher consistencies. The active chlorine conditioning treatment is generally performed at room or slightly elevated temperatures as excessive losses of reagent, are encountered when operating at elevated temperatures or at temperatures near the boiling point.

In many instances, treatment of pulps containing lignin with the small amounts of chlorine, and under the conditions herein specified, produces an actual darkening of the pulp. However, subsequent treatment with alkaline peroxide produces a bleaching eifect far superior, expressed in terms of brightness increment, than is possible of obtainment with alkaline peroxide without such initial treatment.

The pulp, after the pretreatment with active chlorine, as described above, is thereafter bleached in any conventional way with alkaline hydrogen peroxide. This bleaching may be performed upon the same pulp suspension or the suspension may be partially dewatered prior to the peroxide bleach.

The alkaline peroxide solution employed for bleaching the pulp may be prepared from hydrogen peroxide, such as the so-called 100 volume hydrogen peroxide of commerce, and substantially any alkali, or from the alkaline peroxides such as alkali metal peroxide as described more fully hereinafter. The alkaline peroxide solution is stabilized in any well-known manner as, for instance, by the employment of silicate of soda or magnesium salts, or both. The peroxide concentration may vary from 0.25% to calculated as H20; 100 volume, dry basis, although 1% to 3% H202 100 volume will be most generally employed.

The following examples will serve to illustrate the preferred mode of operation. It will be understood that there are many possible variations of the procedure, for instance, consistencies of pulp, pH of pulp suspension, concentration of reagents employed, temperature and time of treatment, depending upon the pulp stock employed and the end result desired. The examples illustrate the action upon stock, all of which is semichemical pulp. It will be understood, however, that the procedure is equally well adapted for blended stock, i. e., semi-chemical pulps mixed with chemical pulps or groundwood pulps, or both.

Example I A batch of semi-chemical pulp prepared from aspen and containing 11% lignin was, at 1 /2% consistency, treated in an aqueous suspension in the presence of 0.5% active chlorine based on the dry weight of the pulp at a pH of 6.8. The active chlorine was supplied by the employment of sodium hypochlorite. The pulp was agitated in the presence of the sodium hypochlorite for 15 minutes at 30 C.

The brightness of the pulp was reduced by the treatment. The original color of the furnish was 52.9 units as read upon a Hunter reflectometer with Universal blue filter. The pulp, after the hypochlorite conditioning, possessed a coloring oi. 42.9 units.

Example 11' A portion of the hypochlorite conditioned pulp 4 of Example I was dewatered to a consistency of 15% and treated for 5 hours in an environment containing alkaline hydrogen mroxide and at a temperature of 50 C. The amount of chemicals, based on the weight of dry pulp, was as follows:

Hydrogen peroxide, 2% of volume H2O: Sodium silicate, 8% 42 B.

Caustic soda, 1.5%

Magnesium sulfate, 0.05%

A hand sheet made from the so treated pulp possessed a brightness of 62.3 as contrasted with a sample of pulp similarly treated with peroxide but not treated with hypochlorite, which possessed a brightness of 61.4.

Example III A portion of the hypochlorite conditioned pulp of Example I was dewatered to a consistency of 25% and was treated at room temperature for 14 days in an environment containing an alkaline peroxide. The chemicals employed in the treatment, based on the weight of oven dried pulp, were as follows:

Hydrogen peroxide. 2% of 100 volume H202 Sodium silicate, 1% 42 B. Trisodium phosphate (Na3PO4.12H2O) 2% Hand sheets prepared from the so treated pulp possessed a Hunter brightness of 61.0 as contrasted with a pulp not treated with hypochlorite but similarly treated with the alkaline peroxide which possessed a brightness of 60.1.

Example IV A portion of the hypochlorite treated pulp of Example I was dewatered to a consistency of 25% and was treated at 55 C. for 4 hours in an environment containing an alkaline peroxide bleaching agent. The chemicals employed in the alkaline peroxide, based on the weight of oven dried pulp were as follows:

Hydrogen peroxide, 2% of 100 volume H20: Sodium silicate, 8% 42 B.

Caustic soda. 2

Magnesium sulfate, 0.05

Hand sheets prepared from the pulp so treated possessed a brightness of 66.1 expressed in Hunter units, in contrast with a brightness of 64.9 for pulp similarly treated with peroxide but not treated initially with the hypochlorite in Example I.

Although hydrogen peroxide has been illustrated as the preferred peroxide, it will be understood that the alkaline peroxide may be obtained by suitably neutralizing with mineral acid an aqueous solution of the alkali metal peroxide such as sodium peroxides or the alkaline earth metal peroxides or the water soluble perborates, all of which furnish hydrogen peroxide in water solution.

In general, the alkaline peroxide solution may be made alkaline with caustic soda, sodium silicate, the alkaline phosphates, such as trisodium phosphate or sodium pyrophosphate. In addition, the alkaline peroxide solution may contain peroxide stabilizers such as sodium silicate or sodium pyrophosphate and may have wetting agents included therein, if desired.

What is claimed is:

The method Of bleaching a semi-chemical pulp containing from about 7% to 17% lignin whichcomprises treating the pulp in an aqueous environment containing 0.25% to 1.0% active chloaaamaa rine based on the weight of dry pulp and at a pH in the range pH 3.5 to 9.0 and thereafter bleaching the pulp with alkaline peroxide.

ROBERT L. MCEWEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 848,361 Gagedois Mar. 26, 1907 1,768,820 Bradley July 1, 1930 1,953,191 Richter Apr. 3, 1934 2,147,618 Rawling Feb. 14, 1939 0 Number Name Date 2,187,016 Craig Jan. 16, 1940 2,359,891 Brallier Oct. 10, 1944 2,465,327 McEwen Mar. 22, 1949 OTHER REFERENCES 

